You're not even using real terminology.
Snapping on an axis:
Snap on X - You select a series of vertecies and snap them onto the X, making them all have the same X coordinate. (Height)
Snap on Y - You select a series of vertecies and snap them onto the Y, making them all have the same Y coordinate. (Length)
Snap on Z - You select a series of vertecies and snap them onto the Z, making them all have the same Z coordinate. (Depth)
In this case, I believe you ought to snap part of the door (the part that is most flat, and with most
flatshading triangulation) onto the Y axis. Then move the upper vertecies on the Y towards the rest of the vehicle.
Does this clear up my meaning? Because in Milkshape there is an ACTUAL TOOL that allows you to Flatten on an Axis, but I believe that SNAP would be better.
Plus, I am 100% sure that there is a snap on axis tool in Blender