Heavy hydrocarbon fuel containing high metal concentrations and all of the soot which is rich in metals that is produced in the system, are feedstocks in a partial oxidation process for the simultaneous continuous production of two streams of cleaned, raw synthesis gas having high and low H.sub.2 O/dry gas mole ratios, respectively. In the process, all of the stream of hot, raw synthesis gas containing entrained particulate carbon and ash that is produced in a first gas generator is quench cooled and scrubbed with water in a quench tank. Simultaneously, split streams of hot raw synthesis gas containing entrained particulate carbon and ash are produced in a second gas generator. One split gas stream is quench cooled and scrubbed with water in a quench tank while the other split gas stream is simultaneously cooled in a convection-type gas cooler and then scrubbed with water. All of the soot recovered from the quench cooling and scrubbing waters in the process is recycled to the first gas generator as a portion of the reactant fuel feed. Fouling and plugging of the tubes of a convection-type gas cooler associated with the second gas generator is prevented.