A system and process for gasifying carbonaceous feedstock with staged slurry addition in order to prevent the formation of tar that causes deposition problems. Dry solid carbonaceous material is partially combusted, then pyrolyzed along with a first slurry stream comprising carbonaceous material in two separate reactor sections, thereby producing mixture products comprising synthesis gas. The second slurry stream comprising particulate carbonaceous material is fed to a drying unit downstream of a heat recovery zone along with the mixture product exiting the heat recovery zone. The resulting final temperature of the second stage mixture products and dried particulate carbonaceous material is between 450° F. and 550° F., a temperature range that is typically not conducive to the emission of heavy molecular-weight tar species.