NOVELTY - A gas mixture (201) containing major proportion of methane is cooled and fed to separation membrane. Residue stream (211) is cooled, and pressure is reduced to provide a nitrogen-enriched vapor and methane-rich liquid (278). Vapor is condensed and fed to distillation column (281) along with methane-rich liquid. Nitrogen withdrawn from top of column is warmed. Liquid methane withdrawn from bottom is pressurized and vaporized. The resulting stream is warmed and combined with permeate stream to provide methane product stream. Thus, gaseous mixture is purified.    USE - Purification of gas mixture containing major proportion of methane and minor proportion of nitrogen.    ADVANTAGE - High purity gaseous methane is efficiently produced from gas mixture.    DETAILED DESCRIPTION - A gas mixture containing major proportion of methane and minor proportion of nitrogen is cooled and fed to a gas separation membrane to provide a permeate stream (210) further enriched in methane and a residue stream further enriched in nitrogen. The residue stream is cooled, and pressure is reduced to provide a nitrogen-enriched vapor and a methane-rich liquid. The nitrogen-enriched vapor is condensed and fed to distillation column along with methane-rich liquid. Gaseous nitrogen withdrawn from top of the distillation column is warmed to provide a gaseous nitrogen product stream (269). Liquid methane withdrawn from bottom of distillation column is pressurized and vaporized to provide a stream of vaporized methane. The resulting stream is warmed and combined with permeate stream to provide a gaseous methane product stream. Thus, gaseous mixture is purified. An INDEPENDENT CLAIM is included for gas mixture purification system.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of distillation system.    Gaseous mixture (201)    Membrane separation unit (208)    Permeate stream (210)    Residue stream (211)    Nitrogen rich stream (269)    Liquid methane rich stream (278)    Distillation column (281)