  We present CO velocity fields and rotation curves for a sample of nearby
galaxies, based on data from the HERACLES survey. We combine our data with
literature THINGS, SINGS and KINGFISH results to provide a comprehensive sample
of mass models of disk galaxies inclusive of molecular gas. We compare the
kinematics of the molecular (CO from HERACLES) and atomic (${\rm H{\scriptstyle
I}}$ from THINGS) gas distributions to determine the extent to which CO may be
used to probe the dynamics in the inner part of galaxies. In general, we find
good agreement between the CO and ${\rm H{\scriptstyle I}}$ kinematics with
small differences in the inner part of some galaxies. We add the contribution
of the molecular gas to the mass models in our galaxies by using two different
conversion factors $\mathrm{\alpha_{CO}}$ to convert CO luminosity to molecular
gas mass surface density - the constant Milky Way value and the radially
varying profiles determined in recent work based on THINGS, HERACLES and
KINGFISH data. We study the relative effect that the addition of the molecular
gas has upon the halo rotation curves for Navarro-Frenk-White (NFW) and the
observationally motivated pseudo-isothermal halos. The contribution of the
molecular gas varies for galaxies in our sample - for those galaxies where
there is a substantial molecular gas content, using different values of
$\mathrm{\alpha_{CO}}$ can result in significant differences to the relative
contribution of the molecular gas and, hence, the shape of the dark matter halo
rotation curves in the central regions of galaxies.
