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  <xs:element id="ngs_ContractWithCustomerRevenueRecognizedIncludingOpeningBalance" abstract="false" name="ContractWithCustomerRevenueRecognizedIncludingOpeningBalance" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_PaymentsForOtherLongTermLiabilitiesNet" abstract="false" name="PaymentsForOtherLongTermLiabilitiesNet" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_LineOfCreditFacilityNonExceedingCapOfComponentCapacity" abstract="false" name="LineOfCreditFacilityNonExceedingCapOfComponentCapacity" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_EmployeeServiceShareBasedCompensationNonvestedAwardsStockOptionsWeightedAverageGrantDateFairValueRollForward" abstract="true" name="EmployeeServiceShareBasedCompensationNonvestedAwardsStockOptionsWeightedAverageGrantDateFairValueRollForward" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="ngs_InventoryWriteOffs" abstract="false" name="InventoryWriteOffs" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_ProductSalesMember" abstract="true" name="ProductSalesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="ngs_PerformanceShares2024ProRatedMember" abstract="true" name="PerformanceShares2024ProRatedMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ngs_RentalEquipmentNet" abstract="false" name="RentalEquipmentNet" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_LineOfCreditFacilityDefaultTriggerCertainDefaultsOfOtherCompanyIndebtednessAmount" abstract="false" name="LineOfCreditFacilityDefaultTriggerCertainDefaultsOfOtherCompanyIndebtednessAmount" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_RentalContractsExcludingLargeHorsepowerCompressorsMember" abstract="true" name="RentalContractsExcludingLargeHorsepowerCompressorsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="ngs_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract" abstract="true" name="ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="ngs_FederalFundRateMember" abstract="true" name="FederalFundRateMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="ngs_InventoryValuationReservesNoncurrent" abstract="false" name="InventoryValuationReservesNoncurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_RentalEquipmentGross" abstract="false" name="RentalEquipmentGross" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_InventoryValuationReservesRollForward" abstract="true" name="InventoryValuationReservesRollForward" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="ngs_LineOfCreditFacilityFixedChargeCoverageRatio" abstract="false" name="LineOfCreditFacilityFixedChargeCoverageRatio" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:pureItemType"/>
  <xs:element id="ngs_AccountsReceivableExcludingContractsWithCustomerAssetCurrent" abstract="false" name="AccountsReceivableExcludingContractsWithCustomerAssetCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_InventoryAllowanceAccrual" abstract="false" name="InventoryAllowanceAccrual" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriodIntrinsicValue" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriodIntrinsicValue" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_RentalEquipmentAccumulatedDepreciation" abstract="false" name="RentalEquipmentAccumulatedDepreciation" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_LineOfCreditFacilityDefaultTriggerRenderingOfCertainJudgmentsAmount" abstract="false" name="LineOfCreditFacilityDefaultTriggerRenderingOfCertainJudgmentsAmount" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedNumberOfSharesToBeTransferred" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedNumberOfSharesToBeTransferred" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="ngs_WeightedAverageSharesOutstandingBasicAndDilutedAbstract" abstract="true" name="WeightedAverageSharesOutstandingBasicAndDilutedAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueGrantsInPeriod" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueGrantsInPeriod" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_EarningsLossPerCommonShareAbstract" abstract="true" name="EarningsLossPerCommonShareAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="ngs_LineOfCreditFacilityLeverageRatio" abstract="false" name="LineOfCreditFacilityLeverageRatio" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:pureItemType"/>
  <xs:element id="ngs_RangeTwoMember" abstract="true" name="RangeTwoMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="ngs_NewCreditAgreementMember" abstract="true" name="NewCreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="ngs_EmployeeServiceShareBasedCompensationNonvestedAwardsStockOptionsOutstandingRollForward" abstract="true" name="EmployeeServiceShareBasedCompensationNonvestedAwardsStockOptionsOutstandingRollForward" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="ngs_RangeOneMember" abstract="true" name="RangeOneMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="ngs_InventoryNetOfReserves" abstract="false" name="InventoryNetOfReserves" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsCancelledOrForfeitedInPeriodIntrinsicValue" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsCancelledOrForfeitedInPeriodIntrinsicValue" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_LineOfCreditFacilityAmendmentFees" abstract="false" name="LineOfCreditFacilityAmendmentFees" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_LineOfCreditFacilityPotentialIncreaseInBorrowingCapacity" abstract="false" name="LineOfCreditFacilityPotentialIncreaseInBorrowingCapacity" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_NumberOfIndependentDirectors" abstract="false" name="NumberOfIndependentDirectors" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="ngs_LineOfCreditFacilityCurrentBorrowingCapacityComponentPercentOfEligibleAccountsReceivableOwingByNonInvestmentGradeDebtors" abstract="false" name="LineOfCreditFacilityCurrentBorrowingCapacityComponentPercentOfEligibleAccountsReceivableOwingByNonInvestmentGradeDebtors" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="ngs_OtherPartsRebuildsMember" abstract="true" name="OtherPartsRebuildsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="ngs_LineOfCreditFacilityPotentialMaximumBorrowingCapacity" abstract="false" name="LineOfCreditFacilityPotentialMaximumBorrowingCapacity" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_TransferOfRentalEquipmentToInventory" abstract="false" name="TransferOfRentalEquipmentToInventory" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodIntrinsicValue" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodIntrinsicValue" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="ngs_IncreaseDecreaseInDeferredIncomeTaxLiabilitiesDueToChangeInTaxLaw" abstract="false" name="IncreaseDecreaseInDeferredIncomeTaxLiabilitiesDueToChangeInTaxLaw" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
</xs:schema>
