A rapid method has been proposed for calculating and predicting the thermal characteristics of chemical processes occurring, in particular, during the hydroconversion of heavy petroleum feedstock in a slurry reactor. The method is based on assessment of the enthalpy change (Delta H) of feedstock components and reaction products, with the formation enthalpies of molecules being presented as a linear function of the number of moles of the elements C, H, N, O, and S. The results of computational study show that this rapid method makes it possible to determine with sufficient accuracy the net heat of reactions on the basis of experimental data on the elemental composition of the feedstock and products.