  We study the dynamics of a quench-prepared domain wall state released into a
system whose unitary time evolution is dictated by the Hamiltonian of the
Heisenberg spin-1/2 gapped antiferromagnetic chain. Using exact wavefunctions
and their overlaps with the domain wall state allows us to describe the release
dynamics to high accuracy, up to the long-time limit, for finite as well as
infinite systems. The results for the infinite system allow us to rigorously
prove that the system in the gapped regime ($\Delta >1$) cannot thermalize in
the strict sense.
