A gas separation method comprising: using a gas separation module (100, 200, 300, 400) of an aircraft fuel tank flammability reduction system, the gas separation module including a housing (102, 232, 332, 402) and a gas separation membrane within the housing, the gas separation membrane having a feed end and a product end with a retentate interior side and a permeate exterior side between the feed end and the product end; supplying a gas feed into the housing, the gas feed containing a first gas and a different second gas; flowing the gas feed along a feed flow path within the housing directionally from the product end to the feed end of the gas separation membrane, the gas feed being segregated from contact with the permeate exterior side; after the feed flow path, flowing the gas feed along a membrane flow path defined by the gas separation membrane from the feed end to the product end of the gas separation membrane, the feed flow path being counter to the membrane flow path; and contacting the retentate interior side with the gas feed in the membrane flow path, permeating at least some of the first gas from the gas feed through the gas separation membrane to the permeate exterior side, and producing retentate that is enriched in the second gas as a result of removing at least some of the first gas from the gas feed; wherein the gas separation module is an air separation module the gas feed comprises an air feed, the first gas comprises oxygen, the second gas comprises nitrogen and the method further comprises: supplying the retentate, which contains nitrogen-enriched air, to a fuel tank for an aircraft; providing a configuration for the gas separation module, selecting one or more process conditions, and thereby compensating for at least a portion of a temperature drop in the gas separation membrane due to enthalpy of gas separation, the one or more process conditions comprising a flow rate; and establishing a temperature of the product end of the gas separation membrane that is within 10°F (6°C) of a temperature of the feed end of the gas separation membrane; and, exchanging heat between the feed flow path and the membrane flow path as enabled by the feed flow path being counter to the membrane flow path and, as a result, increasing a separation efficiency of the gas separation module. using a gas separation module (100, 200, 300, 400) of an aircraft fuel tank flammability reduction system, the gas separation module including a housing (102, 232, 332, 402) and a gas separation membrane within the housing, the gas separation membrane having a feed end and a product end with a retentate interior side and a permeate exterior side between the feed end and the product end; supplying a gas feed into the housing, the gas feed containing a first gas and a different second gas; flowing the gas feed along a feed flow path within the housing directionally from the product end to the feed end of the gas separation membrane, the gas feed being segregated from contact with the permeate exterior side; after the feed flow path, flowing the gas feed along a membrane flow path defined by the gas separation membrane from the feed end to the product end of the gas separation membrane, the feed flow path being counter to the membrane flow path; and contacting the retentate interior side with the gas feed in the membrane flow path, permeating at least some of the first gas from the gas feed through the gas separation membrane to the permeate exterior side, and producing retentate that is enriched in the second gas as a result of removing at least some of the first gas from the gas feed; wherein the gas separation module is an air separation module the gas feed comprises an air feed, the first gas comprises oxygen, the second gas comprises nitrogen and the method further comprises: supplying the retentate, which contains nitrogen-enriched air, to a fuel tank for an aircraft; providing a configuration for the gas separation module, selecting one or more process conditions, and thereby compensating for at least a portion of a temperature drop in the gas separation membrane due to enthalpy of gas separation, the one or more process conditions comprising a flow rate; and establishing a temperature of the product end of the gas separation membrane that is within 10°F (6°C) of a temperature of the feed end of the gas separation membrane; and, exchanging heat between the feed flow path and the membrane flow path as enabled by the feed flow path being counter to the membrane flow path and, as a result, increasing a separation efficiency of the gas separation module. supplying the retentate, which contains nitrogen-enriched air, to a fuel tank for an aircraft; providing a configuration for the gas separation module, selecting one or more process conditions, and thereby compensating for at least a portion of a temperature drop in the gas separation membrane due to enthalpy of gas separation, the one or more process conditions comprising a flow rate; and establishing a temperature of the product end of the gas separation membrane that is within 10°F (6°C) of a temperature of the feed end of the gas separation membrane; and, exchanging heat between the feed flow path and the membrane flow path as enabled by the feed flow path being counter to the membrane flow path and, as a result, increasing a separation efficiency of the gas separation module.