An apparatus in which a downflowing high pressure stream of hot raw synthesis gas from the reaction zone of a free flow partial oxidation gas generator at a temperature in the range of about 1800.degree. to 3000.degree. F. is passed through a first gas diversion and residue separation chamber contained in a first pressure vessel where the velocity of the gas stream is reduced, solid material and molten slag are separated by gravity from the gas stream, and the direction is diverted into a thermally insulated side transfer line. The hot gas stream is then passed through a thermally insulated gas-solids impingement separation means contained in a second pressure vessel where its direction and velocity is changed and additional residue is separated. The hot gas stream is then passed upwardly through a radiant cooler where additional solid matter is removed by gravity and the gas temperature is reduced to a temperature in the range of about 900.degree. to 1800.degree. F. By the subject apparatus, combustion residue may be removed from the raw stream of synthesis gas, and its sensible heat may be extracted at maximum temperature to produce by-product steam.