A combined heat exchanger 5 including a flue gas cooler heat-transfer unit 16 supplied with cooling fluid 15 by a supply pump 13 and an upstream GGH heat-transfer unit 20 for circulation of circulating fluid 19 with a downstream GGH 9 by a circulation pump 17 is arranged at an outlet of a gas air heater 4 for heat exchange of flue gas 2 from a boiler body 1 for oxyfuel combustion with recirculation flue gases 34 and 38. A low-low temperature ESP 6 is arranged at an outlet of the combined heat exchanger 5. A heat-exchange-duty adjustment device 21 is arranged to regulate heats exchanged in the heat-transfer units 16 and 20 in the heat exchanger 5 such that at least flue gas temperature at the inlet of the low-low temperature ESP 6 is kept to an inlet set temperature T 1 .