We have shown that a coarse-grained simulation model, combined with the grandreaction method for simulating acid-base equilibria in two-phase systems, can predict the phase stability of polyelectrolyte complexes (PEC) and partitioning of ionic solutes between the PEC phase and the supernatant. We used the pressure-composition protocol to determine the conditions under which the PEC phase coexists in equilibrium with the supernatant of a given composition. Furthermore, we have shown that the pressure-composition protocol should be supplemented by structural analysis of the PEC phase in order to ensure that phase separation does not occur within the simulation box, resulting in polymer-rich and polymer-poor domains.