TECHNOLOGY FOCUS - POLYMERS - Preferred Polymers: The polymer support is polyetherimide, polyether ether ketone or polyacrylonitrile with a molecular weight of 5,000-25,000. The layer is 50-60 microns thick and is a hollow fiber or spiral wound module (preferably with a backing layer). The polyimide is poly(2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane diiminohydroxybenzidine). The membrane is 3-5 mils thick.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Solvents: The solvent is acetone, MEK (methyl ethyl ketone), NMP (N-methylpyrrolidone), N-alkylpiperidone or dialkylacetamide, preferably mainly acetone. The other solvent is water and/or a lower alkanol. EXAMPLE - A casting solution was prepared by dissolving 17 wt.% Ultem 1000 (RTM: polyetherimide) in a 50/50 solvent mixture of NMP and dimethylacetamide. The solution was cast onto a backing layer of a nonwoven polyester fabric with a knife at a thickness of 6 mils. The coated nonwoven fabric was quenched in a bath of water at 11 degreesC for one minute to form a porous polymeric support layer. This layer was immersed in a rinse tank of water at 50 degreesC for 10 minutes and then immersed in an annealing tank of water at 20 degreesC for 3 minutes. Surface water was removed using an air knife on the surface and a vacuum on the back of the support layer. The support layer was then dried at 60 degreesC for 5 minutes.    An asymmetric gas separation membrane layer-forming solution was prepared containing 8 wt.% 6FDA-HAB (poly(2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane diiminohydroxybenzidine)), 70 wt.% acetone, 16.75 wt.% isopropanol and 0.25 wt.% water. The solution had a viscosity of 1075 cp and was cast onto the porous polymeric support layer using a blade at a thickness of 4 mils. The coated layer was left to stand in air for 7 seconds and then immersed for 3 minutes in a tank of water maintained at 100 degreesC. The support layer was then immersed for 10 minutes in a tank of water maintained at 45 degreesC and subsequently immersed for 3-8 minutes in a tank of deionized water maintained at room temperature to form an asymmetric gas separation membrane layer. The membrane layer was dried at 50 degreesC for 20-30 minutes to provide the supported asymmetric gas separation membrane.    The membrane had permeability coefficients (Barrer) for CO2 and CH4 of 7.3 and 0.091 respectively.