Not sure why, but I can't modify/balance this equation
For starters, You have a problem because of a lack of charge balancing. You should also consider that at neutral pH $\ce{H2S}$ is present as $\ce{HS-}$ and $\ce{CH3SH}$ is present as $\ce{CH3S-}$.  Modifying the reaction to account for charge you get:
$$\ce{HCO3- + HS- + CH4 &lt;=&gt; CH3S-}$$ 

This will give you a start.  The other piece that will help is what the $\ce{HCO3-}$ molecule becomes. For the sake of being able to provide an answer I will assume formate ($\ce{HCO2-})$ is formed. Thus the reaction is now:
$$\ce{HCO3- + HS- + CH4 &lt;=&gt; CH3S- + HCO2-}$$ 

We know that the molecule of $\ce{CH4}$ becomes $\ce{CH3S-}$ and the molecule of $\ce{HCO3-}$ becomes $\ce{HCO2-}$ thus to balance the lost oxygen of $\ce{HCO3-}$ and the lost two hydrogens of $\ce{CH4}$ we add water to the products.
$$\ce{HCO3- + HS- + CH4 &lt;=&gt; CH3S- + HCO2- + H2O}$$

This equation serendipitously requires no coefficients to balance.