  We study the dynamical evolution of coupled one- and two-point functions of a
scalar field in the 2PI framework at the Hartree approximation, including
backreaction from out-of-equilibrium modes. We renormalize the 2PI equations of
motion in an on-shell scheme in terms of physical parameters. We present the
Hartree-resummed renormalized effective potential at finite temperature and
critically discuss the role of the effective potential in a non-equilibrium
system. We follow the decay and thermalization of a scalar field from an
initial cold state with all energy stored in the potential, into a fully
thermalized system with a finite temperature. We identify the non-perturbative
processes of parametric resonance and spinodal instability taking place during
the reheating stage. In particular we study the unstable modes in the region
where the vacuum 1PI effective action becomes complex and show that such
spinodal modes can have a dramatic effect on the evolution of the one-point
function. Our methods can be easily adapted to simulate reheating at the end of
inflation.
