  In integrable many-particle systems, it is widely believed that the
stationary state reached at late times after a quantum quench can be described
by a generalized Gibbs ensemble (GGE) constructed from their extensive number
of conserved charges. A crucial issue is then to identify a complete set of
these charges, enabling the GGE to provide exact steady state predictions. Here
we solve this long-standing problem for the case of the spin-1/2 Heisenberg
chain by explicitly constructing a GGE which uniquely fixes the macrostate
describing the stationary behaviour after a general quantum quench. A crucial
ingredient in our method, which readily generalizes to other integrable models,
are recently discovered quasi-local charges. As a test, we reproduce the exact
post-quench steady state of the Neel quench problem obtained previously by
means of the Quench Action method.
