Abstract: A method for recovering sensible heat from a thermally decomposed high temperature hydrocarbon gas product in the form of high pressure steam of from 40 to 100 kg/cm.sup.2 has been discovered. This method comprises pre-cooling at first the thermally decomposed gas product of high temperature by mixing said gas product with sprayed low temperature hydrocarbon oil as a quenching oil to a temperature of about 300.degree. to 400.degree. C.; and then passing the resulting mixture through the tubes of a shell-and-tube type heat exchanger so as to transfer the heat of said mixture into a high pressure water flowing through the space of the shell side thereof to recover a high pressure steam, followed by separating the condensed hydrocarbon oil from the mixture to reuse said oil by recirculation.