  We present a new Monte-Carlo method for estimating the chemical potential of
model polymer systems. The method is based upon the gradual insertion of a
penetrable `ghost' polymer into the system and is effective for large chain
lengths and at high densities. Insertion of the ghost chain is facilitated by
use of an expanded ensemble in which weighted transitions are permitted between
states characterising the strength of the excluded volume and thermal
interactions experienced by the ghost chain. We discuss the implementation and
optimisation of the method within the framework of the bond fluctuation model,
and demonstrate its precision by a calculation of the finite-size corrections
to the chemical potential.
