We present calculations of heats of adsorption for molecular models of adsorption in heterogeneous porous solids via grand canonical Monte Carlo simulations. The molecular models treat the adsorbent as a matrix of particles arranged in a predefined microstructure, and the parameters are chosen to represent the adsorption of simple molecules in silica gel. Three different adsorbent microstructures have been considered, allowing us to study the role of translational order and connectivity among the matrix particles in determining the heat of adsorption. We have calculated separately the contributions from the adsorbate-solid and adsorbate-adsorbate interactions to the heat of adsorption A comparison with experimental data for methane in silica gel is made. During the course of the work we have revisited the derivation of expressions used to determine heats of adsorption and we present a new derivation which is free from some unnecessary assumptions used previously. In addition we test our results for thermodynamic consistency, and the accuracy of different methods for obtaining the isosteric heat from the Monte Carlo simulations is also investigated.