NOVELTY - Gas separation method involves cooling portion of primary liquid containing C2 (ethylene and ethane) and partially combining cooled liquid and partially condensed secondary hydrocarbon-bearing vapor residue for forming tertiary liquid and tertiary residue. A portion of tertiary liquid is cooled and sent to a light fractionation column (52) to condense C2 and heavier components while methane is evaporated to obtain quaternary vapor and quaternary liquid which is heated and fed to a heavy fractionation column (56).    USE - Separating hydrocarbon feed gas into primary fraction containing methane and lighter components and secondary fraction containing C2 and heavier components.    ADVANTAGE - Since the portion of tertiary liquid is cooled before supply to the light fractionation column, mass and heat transfer are enhanced. The cooling provides greater liquefaction of a higher percent of the C2 and heavier hydrocarbon components while the methane is vaporized.    DETAILED DESCRIPTION - Feed gas stream containing methane, C2 and heavier hydrocarbon components is cooled in heat exchanger and directed to a separator to form primary residue vapor and primary liquid containing C2. A portion of the primary liquid is fed to a heavy fractionation column where the liquid is separated into a secondary hydrocarbon-bearing vapor residue and a secondary liquid containing C2. A portion of the primary residue vapor is intimately contacted with cooled tertiary liquid, for separating quaternary residue vapor and quaternary liquid from the contacting device. The primary residue vapor rising in the counter-current contact type fractionation unit of the fractionation column contacts the cooled tertiary liquid supplied downwardly through the column. The light fractionation column provides a theoretical liquid-vapor equilibrium stage.    An INDEPENDENT CLAIM is also included for feed gas separation apparatus which includes:    (a) a separator for separating feed gas,    (b) a heavy fractionation column for separating primary liquid into secondary residue containing lighter hydrocarbons and a secondary liquid and    (c) a light fractionation column with separator for separating quaternary vapor and quaternary liquid.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic flow diagram of the feed gas separation method.    Light fractionation column (52)    Heavy fractionation column (56)