Abstract The statistical mechanics of quantum solitons is formulated within the framework of a real-time quantum-field theory at finite temperature with the condition that the soliton's profile remains stable. The results of our analysis show that the modification of the energies of the thermally excited quantum particles, due to the presence of the solitons induces a temperature-dependent shift of the soliton mass. When the mass shift of the soliton and the mass shift of the quantum particles are considered together with the change in the entropy of the quantum particles, the low-temperature part of the resultant free energy agrees with that obtained by means of the transfer integral method. The method presented here, which we refer to as the renormalized soliton gas method (RSGM), also provides us with an explanation for the discrepancy between the ideal gas phenomenology (IGP) and the transfer integral method in the low-temperature part of the free energy. The conditions under which RSGM manifests the ideal gas nature will be discussed.