An integrated process is disclosed for treating, at the surface, production fluids recovered from the application of in situ hydrovisbreaking to heavy crude oils and natural bitumens deposited in subsurface formations. The production fluids include virgin heavy hydrocarbons, heavy hydrocarbons converted via the hydrovisbreaking process to lighter liquid hydrocarbons, residual reducing gases, hydrocarbon gases, and other components. In the process of this invention, the hydrocarbons in the production fluids are separated into a synthetic-crude-oil product (a nominal butane to 975.degree. F. fraction with reduced sulfur, nitrogen, metals, and carbon residue) and a residuum stream (a nominal 975.degree. F.+ fraction). Partial oxidation of the residuum is carried out to produce clean reducing gas and fuel gas for steam generation, with the reducing gas and steam used in the in situ hydrovisbreaking process.