  We develop a kinetic theory of microcavity polaritons in presence of both
Coulomb and polariton-phonon interaction, obeying particle number conservation.
We study the growth of a macroscopic population of condensed particles in the
lowest polariton state, under steady-state incoherent excitation of higher
energy states. The collective excitation spectrum, resulting from the Coulomb
Hamiltonian treated within the Hartree-Fock-Bogolubov framework, strongly
influences the polariton condensation kinetics. In particular, for values of
the excitation intensity above the condensation threshold, scattering from the
condensate into the collective excitation modes results in strong quantum
fluctuations that deplete the condensate. A numerical evaluation based on a
few-level scheme shows that the condensate fraction is expected to be lower
than 1 even far above threshold. With increasing system size, the role of the
polariton quantum fluctuations becomes dominant, eventually preventing
condensation to occur for system size larger than 100 micron.
