We present a novel application of the expanded ensemble Monte Carlo (EEMC) simulation method to calculation of the chemical potential of nanocolloidal particles in nanocolloid-polymer mixtures. This approach uses an expanded canonical ensemble in which the colloidal particle diameter is an additional ensemble variable, allowed to vary between zero and the maximum colloid size desired. Using a hard-sphere model system, we demonstrate that this approach is superior to the Widom method for calculating chemical potentials in colloid-polymer systems. Specifically the EEMC leads to lower uncertainties and is capable of calculating accurate colloid chemical potentials for particle sizes where Widom insertion fails due to overlap. The EEMC method is applied to calculate the colloid chemical potential for an infinitely dilute colloidal particle (hard-sphere) in a dilute polymer (hard-sphere chain) solution over a wide range of relative sizes, 0.1