<p>Increasing interest in understanding the formation and dynamics of cool coronal condensations like solar prominences leads to complex magneto-hydrodynamical (MHD) simulations which assume a variety of physical processes responsible for energy balance. Formation of cool structures and their maintenance over the observed periods requires detailed treatment of heating/cooling processes of which the radiative ones are critically important. Most of up-to-date models use the so-called optically-thin radiative losses to account for radiative cooling. In this article, we present radiative-transfer simulations which demonstrate the importance of optically-thick line and continuum transitions. We model the process of free relaxation of prominence kinetic temperature towards the radiative equilibrium which demonstrates the formation of condensations in case where the radiative processes dominate the energy balance. We show a grid of isobaric models and how they relax to radiative equilibrium where the radiative losses are balanced by radiative gains. We also compare our results with previous works. Finally we stress the importance of realistic net radiative cooling rates for MHD modeling of cool coronal condensations.</p>