NOVELTY - The method of cooling a feed gas containing e.g. olefin using an extended binary refrigerant containing a mixture of methane and a 3C hydrocarbon, comprises compressing an extended binary refrigerant vapor in a multistage compressor having a first stage and a last stage with a last stage discharge containing 60-90 mol.% of 3C hydrocarbon and then dividing into a liquid coolant stream (3) and a gaseous coolant stream, and cooling to condense a portion of the extended binary refrigerant vapor from the last stage discharge to form a remaining extended binary refrigerant vapor.    USE - The method is useful for cooling a feed gas containing an olefin such as ethylene, hydrogen and methane using an extended binary refrigerant to increase the total refrigeration capacity of an existing olefins plant employing a cascade refrigeration system having separate propylene and ethylene refrigeration systems (all claimed).    ADVANTAGE - The method ensures easy, efficient and economic cooling of a feed gas containing an olefin such as ethylene, hydrogen and methane using an extended binary refrigerant thus effectively increasing the total refrigeration capacity of an existing olefins plant, and can be easily performed in plants utilizing low pressure or high pressure demethanizers.    DETAILED DESCRIPTION - The method of cooling a feed gas containing e.g. olefin using an extended binary refrigerant containing a mixture of methane and a 3C hydrocarbon, comprises compressing an extended binary refrigerant vapor in a multistage compressor having a first stage and a last stage with a last stage discharge containing 60-90 mol.% of 3C hydrocarbon and then dividing into a liquid coolant stream (3) and a gaseous coolant stream, cooling to condense a portion of the extended binary refrigerant vapor from the last stage discharge to form a remaining extended binary refrigerant vapor and a heavy refrigerant stream (12) having a greater percentage of propylene than the selected mixture, separating the heavy refrigerant stream from the remaining extended binary refrigerant vapor in a separator, cooling to condense a portion of the remaining extended binary refrigerant vapor from the separator to form a light refrigerant stream (13), bringing the heavy refrigerant stream and the light refrigerant stream into heat exchange contact with themselves and each other and with the charge gas in the series of heat exchangers, returning the light refrigerant stream and the heavy refrigerant stream to the compressor, and removing the olefin from the cooled feed gas by fractionation. The extended binary refrigerant provides refrigeration within the temperature of -136 degrees C to 25 degrees C. The charge gas and the heavy refrigerant stream is cooled and then heated and vaporized. The light refrigerant stream is first cooled and partially condensed and then vaporized. The light refrigerant stream is partially condensed by the heavy refrigerant stream, and is further condensed through self-refrigeration. The compressor further includes an intermediate stage. A portion of the heavy refrigerant stream is used for cooling an ethylene fractionator reboiler, a deethanizer condenser and a depropanizer condenser. The light refrigerant vapor, at low pressure, is injected into the liquid refrigerant stream entering an ethylene fractionator condenser downstream of a valve regulating flow of the liquid refrigerant stream into the ethylene fractionator condenser. The direct refrigeration duty of the extended binary refrigerant is decreased by heat exchange between an ethane recycle to a cracking heater produced at the bottom of the fractionating tower, after pressure letdown, and a gross overhead vapor flowing from the top of the fractionating tower. The first light hydrocarbon separation tower is a depropanizer comprising two towers at different pressures, and the higher pressure tower overhead vapor is partially condensed in an interreboiler to provide reflux for the higher pressure tower. The charge gas is deethanized, where no deethanizer condenser is provided and reflux for the deethanizer is withdrawn proximate to a feed tray, where a deethanizer gross overhead enters the fractionating tower. An INDEPENDENT CLAIM is included for an extended binary refrigerant.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic flow diagram of a refrigeration system.    Vapor stream (2)    Liquid coolant stream (3)    Lower pressure stream (11)    Heavy refrigerant stream (12)    Light refrigerant stream. (13)