  We use the Polyakov-loop extended two-flavor quark-meson model as a
low-energy effective model for QCD to study the phase diagram in the
$\mu_I$--$T$ plane where $\mu_I$ is the isospin chemical potential. In
particular, we focus on the Bose condensation of charged pions. At $T=0$, the
onset of pion condensation is at $\mu_I={1\over2}m_{\pi}$ in accordance with
exact results. The phase transition to a Bose-condensed phase is of second
order for all values of $\mu_I$ and in the $O(2)$ universality class. The
chiral critical line joins the critical line for pion condensation at a point
whose position depends on the Polyakov-loop potential and the sigma mass. For
larger values of $\mu_I$ these curves are on top of each other. The
deconfinement line enters smoothly the phase with the broken $O(2)$ symmetry.
We compare our results with recent lattice simulations and find overall good
agreement.
