Hydrogen chloride and acetonitrile are both well characterized polar solvents whose constituent molecules either are linear or can be well approximated in such a way. Here we present a theoretical study of their gas-liquid coexistence properties by means of a reference hypernetted chain integral equation. The standard two and three-center Lennard-Jones models are reformulated on the basis of the integral equation prediction of the coexistence curve, so that the new models describe more precisely the experimental critical properties. Extensive Gibbs ensemble Monte Carlo simulations confirm the reliability of the theoretical estimates and the adequacy of the model. Besides, both X-ray and neutron-diffraction structure factors are accurately reproduced, and a comparison with simulation results for a six-center model for CH3CN also endorses the suitability of the three-center linear model for this substance.