A model which describes the amount of coke deposited onto the catalyst during heavy oil conversion in a trickle-bed reactor is presented. The model comprises on one hand the cracking reactions (multicomponent kinetics) and coke formation kinetics (thermal and catalytic coke) and on the other hand the vapor-liquid equilibria of the reaction mixture. The latter aspect proves crucial to describing the distinct maxima of the amount of coke formed as a function of both the hydrogen-to-oil ratio and the temperature in processing heavy vacuum gas oil. The quantitative predictive power of the model is demonstrated.