  We present numerical evidence for the crystallization of magnons below the
saturation field at non-zero temperatures for the highly frustrated spin-half
kagome Heisenberg antiferromagnet. This phenomenon can be traced back to the
existence of independent localized magnons or equivalently flat-band
multi-magnon states. We present a loop-gas description of these localized
magnons and a phase diagram of this transition, thus providing information for
which magnetic fields and temperatures magnon crystallization can be observed
experimentally. The emergence of a finite-temperature continuous transition to
a magnon-crystal is expected to be generic for spin models in dimension $D>1$
where flat-band multi-magnon ground states break translational symmetry.
