The phrase "assumes away" is correct and can be used in written English.
It means to disregard or ignore something in a given situation or argument. Example: The study assumes away the impact of external factors, such as economic conditions, on the results. This undermines the credibility of their findings.
(The following, for the purposes of explaining the theory, assumes away the fact that coaches can get a hint of what the other side plans by how its players line up. These hints are imperfect, by design.
"By presupposing perfect competition, complete information, and zero transaction costs, neoclassical economics assumes away the need for institutions like courts, parties, and religions to deal with the economic problems that people, firms, and countries face".
The weakness of this type of exercise arises from the assumption of a fixed overall capital output ratio, which assumes away all the vital problems affecting the developing country's capacity to absorb capital and invest its saving in a productive manner.
It thus assumes away the first difficulty.
Focusing particularly on those aspects that neoclassical finance usually assumes away, the (more recent) literature on market microstructure provides a wealth of models featuring effects on market prices that could not be explained in the neoclassical framework.
Traditional economics tends to assume away that basic question.
Proponents of presidential power assume away the existence of Congress.
European unity cannot be achieved by assuming away differences, but by recognising them.
Instead I detect a cheery Vietnam-style faith that obstacles can be assumed away.
The rational-expectations theorists didn't refute Keynesianism: they assumed away the reason for its existence.
The inherent complexity of modern cross-border banking was simply assumed away in the interest of making for good viewing.
