A method and system for recovering CO2 from gasification gas, prevents the recovered CO2 from being contaminated with COS, without repeating cooling and heating operations and without increasing the steam consumption. Gasification gas being produced in a gasifier 10 and containing CO, CO2, COS and H2S is subjected to dust removal in a scrubber 20. Then, a part of the gas is subjected to a CO shift reaction, in which CO is converted into CO2, in a CO shift reactor 30. Another part of the gasification gas is not subjected to the CO shift reaction by means of a bypass 34, and is mixed with the gas after the CO shift reaction. Thereby, the temperature of the mixture gas is set at 180° C. to 300° C., and COS in the mixture gas is converted into H2S in a COS converter 40.