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Exciton condensation indicating the spontaneous formation of electron-hole pair can cause the phase transition from a semimetal to an excitonic insulator by gap opening at the Fermi surface. While the idea of this excitonic insulator has been proposed for decades, current theoretical approaches can only provide qualitative descriptions, and a quantitative predicting tool is still missing. To shed insight on this problem, we developed an ab initio method based on the finite-temperature density functional theory and many-body perturbation theory to compute the exciton condensation critical behavior. Applying our approach to the monolayer TiSe₂, we find a lattice distortion accompanied by the formation of the excitonic gap via electron-phonon coupling without phonon softening, proving that the exciton condensation is the origin of the charge-density-wave state observed in this compound.