We present theoretical calculations of optical absorption-gain spectra for condensed excitons in both three dimensional (3D), 2D, and quasi-2D systems (appropriate for semiconductor quantum wells) as functions of the electron-hole pair density and temperature. The ladder diagram contribution to the vertex function (exciton effect) is included in our calculation. We found that fluctuations cannot destroy the condensation of excitons in 2D at finite temperatures as opposed to a theory of free bosons and the case of Cooper pairs in superconductivity. Such a difference is attributed to the fact that electrons and holes carry opposite charges as opposed to the same charges carried by the Cooper pairs in the case of superconductivity. Our studies show that the effects of exciton Bose condensation on the absorption-gain spectra remain present for temperatures up to 130 K for a 2D system with exciton binding energy of 30{endash}40 meV (appropriate for ZnSe systems). {copyright} {ital 1996 The American Physical Society.}