The various contributions to the configurational entropy, S-conf, and heat capacity have been calculated within the precepts of the polymer chain statistics in the lattice-hole model for a liquid undergoing isothermal polymerization and remaining in an equilibrium state. Out of the five contributions from (i) the fractional occupancy of holes, (ii) coordination number or number of nearest neighbors, (iii) extent of polymerization, (iv) bond flexibility, and (v) entropy of mixing of entities, all but (i) shows a maximum in their plots against the extent of reaction, and of these only (i) and (iv) are temperature dependent. Thus the net S-conf and configurational heat capacity of a polymerizing liquid show a maximum in their plots against the extent of reaction. This maximum varies with the temperature of polymerization. If the polydispersity of a high temperature equilibrium state is frozen-in upon quenching the system to 0 K, the temperature-independent parts of the entropy persists at 0 K, as for isotopic mixtures of elements and chemical compounds. In real systems, this may occur if the liquid is impure or its molecules interact with its container's wall, thus preventing the formation of an infinitely long chain in the equilibrium state near 0 K. Calculations are also done for the monodispersed system and modifications made to the approximations used in the earlier calculations. These lead to a decrease in S-conf to zero at 0 K for a system with an infinite long polymer chain. The conclusions agree with the experimental finding that the net heat capacity of a polymerizing liquid at a fixed temperature first increases slowly, and then decreases rapidly, thus showing a broad maximum, before configurational freezing vitrifies the melt. It is predicted that if polymerization is done at a sufficiently high temperature so that the ultimately polymerized state is a liquid, the plots of the experimental heat capacity against the extent of polymerization will show a broad asymmetric maximum. Implications of these findings for our current understanding of vitrification and energy landscape models are discussed.