Abstract: Carbon dioxide is separated from a flue gas produced in a combustion plant. Flue gas is supplied to a membrane unit having at least one membrane module provided with a membrane that is selective for CO2. A portion of the CO2 is separated from the flue gas in the module, producing a CO2-enriched permeate. CO2-depleted flue gas remaining on the retentate side of the module is supplied to at least one additional CO2 separating unit and a portion of the CO2 in the retentate is separated by an absorbent. The result is a reduction in energy consumption.