  We report a numerical study, supplemented by phenomenological explanations,
of ``energy condensation'' in forced 2D turbulence in a biperiodic box.
Condensation is a finite size effect which occurs after the standard inverse
cascade reaches the size of the system. It leads to emergence of a coherent
vortex dipole. We show that the time growth of the dipole is self-similar, and
it contains most of the injected energy, thus resulting in an energy spectrum
which is markedly steeper than the standard $k^{-5/3}$ one. Once the coherent
component is subtracted, however, the remaining fluctuations have a spectrum
close to $k^{-1}$. The fluctuations decay slowly as the coherent part grows.
