<<Disclaimer: Verify this information before applying it to your situation.>>
 
The transition of water to vapor and vice versa involves a lot of energy.
For this reason, the major mechansim* of transfer of heat in a slice of
bread being cooled/warmed is the evaporation/condensation of water.  In a
zip-lock bag the water can't escape so it condenses on the inside of the
bag and collects.
 
Solution:  Let the bread cool to refridgerator temperature outside the bag
(or with it open) then seal it in the bag.  There should then be no excess
moisture in the bag.      If the bread is then warmed to room temperature
inside the sealed bag, this will leave it dry.   The solution to that is to
let it warm outside the bag.   Moisture from the air (about equal to the
amount evaporated in the fridge) will condense on the bread as it warms,
remoisturizing it.  This exposes the bread to fungi, etc. in the air, so it
should be done shortly before eating.  Both cooling and warming will take
much longer for a loaf than a slice.
 
Another way to maintain moisture content in bread is to use honey instead
of sugar when baking it (about 1/2 cup honey for each cup of sugar).
Honey has hydroscopic properties and is said to give bread better storage
(does't go stale as fast).
 
_____________
*   As opposed to convection which transfers much less energy as it does
not involve a phase change and as air (considered separately from its
moisture content) has low heat capacity.