Impact of non-LTE effect on the emission of the 5_1,4–4_1,3 transition of methanol (E_up = 49.7 K, A_ij = 6.0 × 10^–5) emitted from the inner 2″ of the envelope. The j-axis indicates the ratio of the integrated line flux computed with Ratran under non-LTE assumptions and LTE assumptions. The non-LTE computation was made with a consistent calculation of the population levels of methanol, including collision with H₂ and radiative pumping. The line flux from was computed assuming the same collisional rate coefficients and an isotopologue ratio of ¹⁶O/¹⁸O = 560.