  We derive the nonequilibrium real-time evolution of an O(N) - invariant
scalar quantum field theory in the presence of a nonvanishing expectation value
of the quantum field. Using a systematic 1/N expansion of the 2PI effective
action to next-to-leading order, we obtain nonperturbative evolution equations
which include scattering and memory effects. The equivalence of the direct
method, which requires the resummation of an infinite number of skeleton
diagrams, with the auxiliary-field formalism, which involves only one diagram
at next-to-leading order, is shown.
