Abstract: The invention relates to a process and apparatus for performing steam reforming and water gas shift reaction. Steam reformer product gas comprising H2O and CO is introduced into a combo-boiler which comprises a shell and tube reactor having at least two tube zones and a common shell zone. One of the tube zones is a shift reaction zone wherein the tubes are filed with a shift reaction catalyst. In this shift reaction zone H2O and CO are converted into CO2 and H2. Cooling medium flowing through the shell side of the combo-boiler maintains the shift reaction zone under substantially isothermal conditions. Another of the tube zones is a first process gas cooling zone wherein the cooling medium undergoes indirect heat exchange with a first process gas, for example, the steam reformer product gas before it is introduced into the shift reaction zone.