An oxyfuel combustion boiler system comprising a boiler body for oxyfuel combustion, a gas air heater for heat exchange of flue gas from said boiler body with recirculation flue gas (34,38), characterized in that, said boiler system further comprises: a combined heat exchanger arranged at an outlet of said gas air heater, a low-low temperature electrostatic precipitator arranged at an outlet of said combined heat exchanger, a downstream GGH arranged downstream of said low-low temperature electrostatic precipitator, a flue gas cooler heat-transfer unit arranged in said combined heat exchanger, an upstream GGH heat-transfer unit arranged in said combined heat exchanger, a supply pump for supplying cooling fluid to said flue gas cooler heat-transfer unit, a circulation pump for circulation of circulating fluid between said downstream GGH and said upstream GGH heat-transfer unit, and a heat-exchange adjustment device for adjusting heat exchange in said flue gas cooler and upstream GGH heat-transfer units (16,20) to keep at least a temperature of the flue gas at an inlet of the low-low temperature electrostatic precipitator to an inlet set temperature (T 1), wherein said heat-exchange adjustment device comprises first and second inner vanes (22,23) for independently regulating flow rates of the flue gas flowing to said flue gas cooler and upstream GGH heat-transfer units (16,20), a cooling-fluid bypass valve for flowing of the cooling fluid with said flue gas cooler heat-transfer unit being bypassed, a circulating-fluid bypass valve for bypassing said upstream GGH heat-transfer unit to return circulating fluid back to the downstream GGH, an inlet thermometer for detecting a temperature of the flue gas at an inlet of said low-low temperature electrostatic precipitator, an outlet thermometer for detecting a temperature of the flue gas at an outlet of said downstream GGH, a vane opening-degree controller for independently controlling opening degrees of said first and second inner vanes (22,23), an inlet temperature controller for controlling opening degrees of said cooling-fluid and circulating-fluid bypass valves (24,25) such that a detected temperature of the inlet thermometer at the inlet of said low-low temperature electrostatic precipitator is made equal to an inlet set temperature (T 1), an outlet temperature controller for controlling an opening degree of said second inner vane through said vane opening-degree controller such that a detected temperature of the outlet thermometer at the outlet of said downstream GGH is made equal to an outlet set temperature (T 2), and an operational controller for command-controlling said vane opening-degree controller and said inlet and outlet temperature controllers (29,31) depending on an operational condition. a combined heat exchanger arranged at an outlet of said gas air heater, a low-low temperature electrostatic precipitator arranged at an outlet of said combined heat exchanger, a downstream GGH arranged downstream of said low-low temperature electrostatic precipitator, a flue gas cooler heat-transfer unit arranged in said combined heat exchanger, an upstream GGH heat-transfer unit arranged in said combined heat exchanger, a supply pump for supplying cooling fluid to said flue gas cooler heat-transfer unit, a circulation pump for circulation of circulating fluid between said downstream GGH and said upstream GGH heat-transfer unit, and a heat-exchange adjustment device for adjusting heat exchange in said flue gas cooler and upstream GGH heat-transfer units (16,20) to keep at least a temperature of the flue gas at an inlet of the low-low temperature electrostatic precipitator to an inlet set temperature (T 1), wherein said heat-exchange adjustment device comprises first and second inner vanes (22,23) for independently regulating flow rates of the flue gas flowing to said flue gas cooler and upstream GGH heat-transfer units (16,20), a cooling-fluid bypass valve for flowing of the cooling fluid with said flue gas cooler heat-transfer unit being bypassed, a circulating-fluid bypass valve for bypassing said upstream GGH heat-transfer unit to return circulating fluid back to the downstream GGH, an inlet thermometer for detecting a temperature of the flue gas at an inlet of said low-low temperature electrostatic precipitator, an outlet thermometer for detecting a temperature of the flue gas at an outlet of said downstream GGH, a vane opening-degree controller for independently controlling opening degrees of said first and second inner vanes (22,23), an inlet temperature controller for controlling opening degrees of said cooling-fluid and circulating-fluid bypass valves (24,25) such that a detected temperature of the inlet thermometer at the inlet of said low-low temperature electrostatic precipitator is made equal to an inlet set temperature (T 1), an outlet temperature controller for controlling an opening degree of said second inner vane through said vane opening-degree controller such that a detected temperature of the outlet thermometer at the outlet of said downstream GGH is made equal to an outlet set temperature (T 2), and an operational controller for command-controlling said vane opening-degree controller and said inlet and outlet temperature controllers (29,31) depending on an operational condition.