  We describe a lattice-gas model suitable for studying the generic effects of
polydispersity on liquid-vapor phase equilibria. Using Monte Carlo simulation
methods tailored for the accurate determination of phase behaviour under
conditions of fixed polydispersity, we trace the cloud and shadow curves for a
particular Schulz distribution of the polydisperse attribute. Although
polydispersity enters the model solely in terms of the strengths of the
interparticle interactions, this is sufficient to induce the broad separation
of cloud and shadow curves seen both in more realistic models and experiments.
