You can rewrite the cylindrical equation into the equivalent cartesian equation with a simple replacement
Thread[{r, \[Theta], z} ->
CoordinateTransform["Cartesian" -> "Cylindrical", {x, y, z}]]
As for the cylinder x^2 + y^2 = 100
, it seems to work for me:
cylEq = r^2 + z^2;
coordTransform =
Thread[{r, \[Theta], z} ->
CoordinateTransform["Cartesian" -> "Cylindrical", {x, y, z}]];
cartEq = cylEq /. coordTransform;
SliceContourPlot3D[cartEq,
x^2 + y^2 == 100, {x, -10, 10}, {y, -10, 10}, {z, -10, 10}]