A method for producing carbon dioxide and nitrogen from combustion exhaust gas containing less than about 10% oxygen by weight comprises the steps of (a) treating the exhaust gas to remove particulate matter, (b) compressing the exhaust gas to a pressure in the range from about 25 psia to about 200 psia, (c) purifying the exhaust gas to remove trace contaminants, (d) separating the exhaust gas to produce a carbon dioxide rich fraction and a nitrogen rich fraction, (e) liquefying the carbon dioxide rich fraction and distilling off volatile contaminants to produce pure carbon dioxide, (f) purifying the nitrogen rich fraction to remove contaminants, and (g) cryogenically fractionally distilling the nitrogen rich fraction to produce pure nitrogen and preferably arc argon-enriched fraction which is then separated to produce argon. The combustion exhaust gas may be obtained from an ammonia plant reformer furnace and the nitrogen produced may be employed to synthesise ammonia. The carbon dioxide may be reacted with the ammonia to form urea.