A HEAVY HYDROCARBON STREAM IS CONVERTED IN A HIGHTEMPERATURE, HIGH PRESSURE FLUIDIZED COKER INTO A DISTILLAABLE, LOWER-BOILING HYDROCARBON PRODUCT OIL WHILE PRODUCING A COKE PRODUCT. THE COKE PRODUCT IS CONVERTED IN A HIGH-PRESSURE, HIGH-TEMPERATURE, PARTIAL COMBUSTION REACTOR INTO CARBON MONOXIDE, BY REACTION WITH STEAM AND A SMALL AMOUNT OF SUBSTANTIALLY PURE OXYGEN. THE CARBON MONOXIDE IS CONVERTED INTO CARBON DIOXDE IN A SHIFT REACTOR. THE HYDROGEN PRODUCED IN THE WATER-GAS REACTOR AND THE SHIFT REACTOR IS PREFERABLY USED TO HYDROCRACK THE LOWER-BOILING HYDRO-CARBON OIL PRODUCT FROM THE COKER, OR MAY BE USED FOR OTHER PURPOSES SUCH AS HYDROFINING ECT.