An attractive way to use residual fuels from petroleum refineries (vacuum residue and petcoke) is their gasification to produce syngas, which contains mainly H-2, CO and small quantities of CH4, CO2, as well as nitrogen and sulfur compounds. Syngas has a large range of possibilities to e used, as fuel in IGCC plants, or as raw material for other chemical manufacturing processes An equilibrium based model for entrained-flow gasification of petroleum refinery residual fuels is here proposed. Using an equation oriented (EO) approach where the process model is set up as a Non Linear Algebraic Equations System and simultaneously solved, the model gives the syngas composition, with CO, H-2, CO2, CH4, H2S, HCN, NH3, N-2, O-2, COS, and H2O, as well as the slag composition, with NiS, FeS, V2O3, ashes, and no reacted hydrocarbon. The slag mass flow and composition calculation is an important feature not considered by previously published models. A sensitivity analysis was made in order to show the effect of input variables. Model predictions were compared to published industrial and experimental data. The results are in good agreement, between both the simulated and published data.