NOVELTY - Making supported gas separation molecular sieve membrane (10) comprises applying molecular sieve crystals to surface of porous support to provide seeded porous support surface, contacting seeded porous support surface with synthesis gel mixture under hydrothermal synthesis conditions for shortened crystallization time of greater than or equal to 20 hours to provide coated porous support, and calcining coated porous support under calcination conditions including 250-650 degrees C to provide molecular sieve membrane having carbon dioxide/methane separation selectivity at 138 kPa and 22 degrees C of greater than or equal to 100.    USE - Method for making supported gas separation molecular sieve membrane (claimed).    ADVANTAGE - The method provides sieve membrane has good gas separation characteristics, provides rapid formation upon the surface of a porous substrate of thin layer of molecular sieve crystals that is dense, is free of defects, shortens the crystallization time, and is economical.    DETAILED DESCRIPTION - Making supported gas separation molecular sieve membrane (10) comprises applying molecular sieve crystals to surface of porous support to provide seeded porous support surface, contacting the seeded porous support surface with synthesis gel mixture under hydrothermal synthesis conditions for shortened crystallization time of greater than or equal to 20 hours to provide a coated porous support, and calcining the coated porous support under calcination conditions including 250-650 degrees C to provide the supported gas separation molecular sieve membrane having carbon dioxide/methane separation selectivity at 138 kPa and 22 degrees C of greater than or equal to 100. An INDEPENDENT CLAIM is included for separating first gas component from gas mixture containing two gas components comprising providing gas separation molecular sieve membrane system having feed side with feed inlet a retentate outlet, and permeate side; introducing the gas mixture to feed inlet of the gas separation molecular sieve membrane system; and yielding permeate stream from the permeate side of the gas separation molecular sieve membrane system and retentate stream is depleted of the first gas component and permeate stream is rich in the first gas component. The membrane system is selectively permeable to the first gas component on the second gas component. The retentate stream is depleted of the first gas component. The permeate stream is rich in the first gas component.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross sectional view of the gas separation membrane system.    Gas separation molecular sieve membrane (10)    Inside surface (14)    Outside surface (16)    Tubular conduit (24)    Permeable membrane layer (26)    Feed inlet end (28)