  We report Northern Extended Millimeter Array (NOEMA) CO($J = 3 - 2$)
observations of the dusty star-forming galaxy ACT-S\,J020941+001557 at $z =
2.5528$, which was detected as an unresolved source in the Atacama Cosmology
Telescope (ACT) equatorial survey. Our spatially resolved spectral line data
support the derivation of a gravitational lens model from 37 independent
velocity channel maps using a pixel-based algorithm, from which we infer a
velocity-dependent magnification factor $\mu \approx 7-22$ with a
luminosity-weighted mean $\left<\mu\right>\approx 13$. The resulting
source-plane reconstruction is consistent with a rotating disk, although other
scenarios cannot be ruled out by our data. After correction for lensing, we
derive a line luminosity $L^{\prime}_{\rm CO(3-2)}= (5.53\pm 0.69) \times
10^{10}\,{\rm \,K\,km\,s^{-1}\,pc^{2}}$, a cold gas mass $M_{{\rm gas}}= (3.86
\pm 0.33) \times 10^{10}\,M_{\odot}$, a dynamical mass $M_{\rm dyn}\,{\rm
sin}^2\,i = 3.9^{+1.8}_{-1.5} \times 10^{10}\,M_{\odot}$, and a gas mass
fraction $f_{\rm gas}\,{\rm csc}^2\,i = 1.0^{+0.8}_{-0.4}$. The line brightness
temperature ratio of $r_{3,1}\approx 1.6$ relative to a Green Bank Telescope
CO($J=1-0$) detection may be elevated by a combination of external heating of
molecular clouds, differential lensing, and/or pointing errors.
