  We present a novel method for the accurate numerical determination of the
phase behavior of fluid mixtures having large particle size asymmetries. By
incorporating the recently developed geometric cluster algorithm within a
restricted Gibbs ensemble, we are able to probe directly the density and
concentration fluctuations that drive phase transitions, but that are
inaccessible to conventional simulation algorithms. We develop a finite-size
scaling theory that relates these density fluctuations to those of the
grand-canonical ensemble, thereby enabling accurate location of critical points
and coexistence curves of multicomponent fluids. Several illustrative examples
are presented.
