Supposing that..., Pretending to..., Considering that..., Assuming that...
This expression is used to hypothesize doing some action or to regard some state of affairs as being true. It is mainly used in the form -는 셈치고 to mean hypothetically doing the action in the preceding clause or to consider such a scenario and then actually to do the action in the following clause. It only attaches to verbs.
This expression is used in a manner similar to -(느)ㄴ다고 치다 in that they both posit a hypothetical situation, but they have the following differences.