A methodology based on a combination of the histogram reweighting technique and the fourth order Binder cumulant was developed to determine the critical point in generalized condensation-evaporation systems. This methodology was applied, particularly, to the study of the critical point of the condensation transition for linear molecules adsorbed on square lattices. The results were obtained by means of grand canonical Monte Carlo simulations within the lattice gas model, along with finite size scaling. The method was tested in a system of interacting monomers, in which the critical point can be determined exactly. Furthermore, the application of this method to the determination of the critical point in dimer systems with attractive interactions, gave better results than the previous reported studies. In addition, the adsorption isotherms at different temperatures, as well as the phase diagrams for monomer and dimer systems were obtained, achieving significant improvements in the phase diagram for dimers.