A system is described wherein a boiler 10 is integrated with an oxygen producing device 12. Combustion heat generated in the boiler 10 is used to generate steam in the boiler and is also applied to at least one of a sweep gas stream 13 and a feed gas stream 15 to ensure that the sweep gas and feed gas streams 12, 15 are provided at the appropriate temperature to the oxygen producing device 12. Flue gas generated by fuel combustion within the combustion chamber 14 may be used as the sweep gas stream 13, in which case, the flue gas exiting the oxygen producing device 12 includes the oxygen removed from the feed gas stream. The flue gas/oxygen mixture may be used for fuel combustion within the combustion chamber, and may be provided to an oxygen separator 28 for removing oxygen from the flue gas. Sensible heat contained in the oxygen depleted feed gas from the oxygen producing device 12 may be recovered by the feed gas stream 15.