NOVELTY - A gas separation membrane comprises a carbon molecular sieve, where the gas separation membrane has an nitrogen/methane selectivity of at least about 5.0, and a nitrogen permeability of at least about 3.0 Barrers at 35 degrees C and 65 psia in pure nitrogen and pure methane streams.    USE - The gas separation membrane and method are useful for decreasing the nitrogen content of a nitrogen/methane gas mixture (claimed).    ADVANTAGE - The gas separation membrane has high adaptability, high reliability, low energy consumption, and low capital cost in installation, operation and maintenance.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) forming a nitrogen/methane separation carbon membrane which involves: subjecting a polymer precursor membrane to a pyrolysis temperature of at least about 675 degrees C in an inert atmosphere, where the polymer precursor membrane is a polyimide having at least 60 wt.% carbon content and a hydrogen content of less than 5 wt.%;    (2) a gas enrichment device, which comprises a nitrogen/methane gas stream inlet, a nitrogen-enriched gas stream outlet, a nitrogen-depleted gas stream outlet, and a carbon molecular sieve membrane interposed between the gas stream inlet and the nitrogen-depleted stream outlet on one side of the membrane and the nitrogen-enriched gas stream outlet on the other side, where the carbon molecular sieve has a nitrogen/methane selectivity of at least about 5.0 and a nitrogen permeability of at least about 3 Barrers at 35 degrees C and 65 psia in pure nitrogen and pure methane streams; and    (3) decreasing the nitrogen content of a nitrogen/methane gas mixture, which involves contacting the mixture with a gas separation membrane, and separating two gas streams at the membrane, where the gas separation membrane has a nitrogen/methane selectivity of at least about 5.0 and a nitrogen permeability of at least about 3 Barrers at 35 degrees C and 65 psia in pure nitrogen and pure methane streams.