Abstract: The invention relates to a method for separating a feed gas containing at least 20 Mol % of CO2 and at least 20 Mol % of methane, by partial condensation and/or by distillation, the gas at a pressure of at least 40 bar abs, including expanding at least one portion of the feed gas in a turbine producing an expanded feed stream at a pressure of less than 90 bar abs, separating at least one portion of the expanded feed stream by partial condensation and/or by distillation thus obtaining a CO2-depleted gas and a CO2-enriched liquid, wherein the temperature of the expanded feed gas at the outlet of the turbine is below ?56.6° C., and wherein the process does not use an external refrigeration source; and wherein the CO2-depleted gas is introduced into a supplementary separation step, in order to obtain a stream that is more depleted in CO2 and a CO2-rich stream.