In the boiler plant the flue gases coming from the firing space undergo division into at least two part-currents which are cooled to different degrees by giving up heat to a heat exchange medium and are then mixed together partly for producing a gas current with a desired temperature dependent on the rates of mixing of the two part-currents. The mixed current is then taken up by a heat user. Parts of the flue gases not going to the heat user are run through all heat absorbers of the system as far as the off-gas stack. The amounts of the part-currents of flue gases, used for forming the gas current with the desired temperature of mixing, are controlled dependent on the mixing temperature.