  We investigate the allowed range of reheating temperature values in light of
the Planck 2015 results and the recent joint analysis of Cosmic Microwave
Background (CMB) data from the BICEP2/Keck Array and Planck experiments, using
monomial and binomial inflationary potentials. While the well studied $\phi^2$
inflationary potential is no longer favored by current CMB data, as well as
$\phi^p$ with $p>2$, a $\phi^1$ potential and canonical reheating ($w_{re}=0$)
provide a good fit to the CMB measurements. In this last case, we find that the
Planck 2015 $68\%$ confidence limit upper bound on the spectral index, $n_s$,
implies an upper bound on the reheating temperature of $T_{re}\lesssim 6\times
10^{10}\,{\rm GeV}$, and excludes instantaneous reheating. The low reheating
temperatures allowed by this model open the possiblity that dark matter could
be produced during the reheating period instead of when the Universe is
radiation dominated, which could lead to very different predictions for the
relic density and momentum distribution of WIMPs, sterile neutrinos, and
axions. We also study binomial inflationary potentials and show the effects of
a small departure from a $\phi^1$ potential. We find that as a subdominant
$\phi^2$ term in the potential increases, first instantaneous reheating becomes
allowed, and then the lowest possible reheating temperature of $T_{re}=4\,{\rm
MeV}$ is excluded by the Planck 2015 $68\%$ confidence limit.
