  Methyl cyanide (CH$_3$CN) is one of the most abundant and widely spread
interstellar complex organic molecules (iCOMs). Several studies found that, in
hot corinos, methyl cyanide and methanol abundances are correlated suggesting a
chemical link, often interpreted as a synthesis of them on the interstellar
grain surfaces. In this article, we present a revised network of the reactions
forming methyl cyanide in the gas-phase. We carried out an exhaustive review of
the gas-phase CH$_3$CN formation routes, propose two new reactions and
performed new quantum mechanics computations of several reactions. We found
that 13 of the 15 reactions reported in the databases KIDA and UDfA have
incorrect products and/or rate constants. The new corrected reaction network
contains 10 reactions leading to methyl cyanide. We tested the relative
importance of those reactions in forming CH$_3$CN using our astrochemical
model. We confirm that the radiative association of CH${_3}{^+}$ and HCN,
forming CH$_{3}$CNH$^{+}$, followed by the electron recombination of
CH$_{3}$CNH$^{+}$, is the most important CH$_3$CN formation route in both cold
and warm environments, notwithstanding that we significantly corrected the rate
constants and products of both reactions. The two newly proposed reactions play
an important role in warm environments. Finally, we found a very good agreement
between the CH$_3$CN predicted abundances with those measured in cold ($\sim$10
K) and warm ($\sim$90 K) objects. Unexpectedly, we also found a chemical link
between methanol and methyl cyanide via the CH$_{3}^{+}$ ion, which can explain
the observed correlation between the CH$_3$OH and CH$_3$CN abundances measured
in hot corinos.
