A waste heat boiler includes an axial bypass pipe and multiple heat transfer pipes disposed within a cylindrical jacket. A hot exhaust gas stream transported between inlet and outlet ends of the heat transfer pipes is cooled by a coolant. A control system controls gas passage velocity and quantity of the exhaust gas stream through the bypass pipe whereby the exhaust gas stream exhaust temperature is maintained within a predetermined temperature range. The control system includes a stopper disposed at the outlet end of the bypass pipe and has a head plate extending into the outlet end of the bypass pipe. The stopper is axially displaceable from a closed position, where an outer surface of the stopper head plate contacts an inner surface of the bypass pipe outlet end. The gas passage formed between the inner surface of the bypass pipe outlet end and the outer surface of the stopper head plate has a cross-section that increases as the stopper is axially displaced from the closed position. The stopper is cooled by a cooling medium.