A fluid coking-gasification process for converting heavy hydrocarbonaceous chargestocks to lower boiling products in which an inorganic metal composition is used to mitigate slagging in the gasifier, wherein the metal is selected from the alkaline-earths, the rare earths, and zirconium. The inorganic metal composition is added either directly into the gasifier or it is mixed with the coke passing from the heating zone to the gasification zone.