Try this:
list = {x1 >= 0, x2 >= 0, x3 >= 0, 2 x1 >= x2, 2 x3 >= x2, 2 x1 + x2 <= 4, 2 x3 + x2 <= 4} ; Variables[Part[list,All,1]] Variables[Apply[List,list,1]]
I.M.
