Carbon monoxide (CO) may be removed from flue gas generated by oxyfuel combustion of a hydrocarbon or carbonaceous fuel, by contacting the flue gas, or a CO-containing gas derived therefrom, at a first elevated temperature, e.g. at least 80° C., and at a first elevated pressure, e.g. at least 2 bar (0.2 MPa), with at least one catalyst bed comprising a CO-oxidation catalyst in the presence of oxygen (O2) to convert CO to carbon dioxide and produce carbon dioxide-enriched gas. The carbon dioxide produced from the CO may be recovered from the carbon dioxide-enriched gas using conventional carbon dioxide recovery techniques. NO in the flue gas may also be oxidized to nitrogen dioxide (NO2) and removed using conventional NO2 removal techniques, or may be reduced in the presence of a reducing gas to nitrogen (N2) which does not have to be removed from the gas.