In a process of reacting a hydrocarbon with steam and/or carbon dioxide in a reaction zone heated by a combustion furnace to give olefins or process gas containing carbon oxides and hydrogen high grade heat is recovered from furnace flue gases and/or process gas whereby to cool such gases to 150-300 DEG C. When the process gas is used in a synthesis of for example methanol or ammonia, gases at 150 DEG -300 DEG C are also produced. Previously the recovery of heat from such low grade heat sources has been inefficient or inconvenient. According to the invention the heat is recovered by means of an intermediary liquid coolant, which is brought into direct heat exchange with streams to be used in the process. Preferably the liquid is water under pressure and is brought into direct heat exchange with a gas to be fed to the process; by this means 10-30% of the steam can be provided from low grade waste heat.
