Reaction mechanism for reduction of alkyl halide to alkane
The action of HCl on Zn results in the formation of hydrogen. 

Next, there is some surface chemistry where the diffusion of atomic hydrogen (or hydrogen radical) occurs with select metals. See, for example, this 2008 thesis "Alkaline dissolution of aluminum: surface chemistry and subsurface interfacial phenomena", by Saikat Adhikari, who cites the "ingress of atomic hydrogen into the metal". 

Then, the action of a hydrogen radical on an alkyl halide:

$\ce{.H + CH3Cl -&gt; HCl + .CH3}$  (Source on radical reactions)

$\ce{.H + .CH3 -&gt; CH4}$

I suspect more reactions are possible resulting in minor products, for example:

$\ce{.CH3 + .CH3 -&gt; C2H6}$

$\ce{.H + CH4 -&gt; H2 + .CH3}$

......

The final products include HCl, CH4 and other minor products.