  We compute the gravitational production of conformally coupled superheavy
particles during the phase transition from the end of inflation to the
beginning of kination for $\alpha$-attractors potentials in the context of
Quintessential Inflation ($\alpha$-QI), showing that the maximum value of the
reheating temperature, independently of the value of the parameter $\alpha$, is
near $10^9$ GeV. This result, which contradicts the usual belief that the
reheating via the production of superheavy massive particles leads to an
inefficient reheating temperature, is due to the fact that in our numerical
calculations we take into account the contribution of the large wavelength
modes to the reheating temperature, which never happens in analytical
calculations where only ultraviolet modes are considered.
