The invention relates to a method for fractionating a gas produced by the pyrolysis of hydrocarbons (1) for the recovery of C.sub.2 and higher hydrocarbons, in particular ethylene and ethane, comprising an operation (70) for compressing and drying gas (1) as a result of it passing into a series of compressors (3, 10, 17, 24, 36), an operation (80) for progressively cooling and partially liquefying the gas produced by compression and drying operation (70) as a result of it successively passing into a cryogenic exchanger (62), and a distillation operation (90) in a column (64), and the corresponding installation.According to the invention, cryogenic exchanger (62) is cooled by at least one cooling liquid (400) which is liquid natural gas (LNG).