  A new treatment of the critical point of the two-dimensional interacting Bose
gas is presented. In the lowest order approximation we obtain the critical
temperature T_c ~ 2 \pi n/[ m \log (2\pi/mg)], where n is the density, m the
mass, and g the coupling. This result is based on a new formulation of
interacting gases at finite density and temperature which is reminiscent of the
thermodynamic Bethe ansatz in one dimension. In this formalism, the basic
thermodynamic quantities are expressed in terms of a pseudo-energy. Consistent
resummation of 2-body scattering leads to an integral equation for the
pseudo-energy with a kernel based on the logarithm of the exact 2-body
S-matrix.
