In the history of the United States of America, two presidents have been successfully impeached by the House of Representatives, Andrew Johnson and Bill Clinton, but both were later acquitted by the Senate. Richard Nixon, on the other hand, was not impeached, because he resigned the presidency shortly before the House was due for the vote. In conclusion, no president has ever been forcefully removed from office through impeachment.
The second and last way for a president to be removed from office is granted to the rest of the executive branch and Congress by the 25th amendment. If the vice president and majority of Cabinet or Congress see the President as unfit to serve, the Vice President becomes Acting President. Once the real President says he/she is able to serve, he/she becomes President again. Finally, in order to fully remove the President from office, there must be a 2/3rds majority in Congress in favor of this proposition to do so.
Overall, impeachment is a much more plausible way to have a president removed from office while he/she is capable of governing, as the 25th amendment was primarily meant for health issues (but this amendment would make for a fascinating part of a tv show where the VP and other cabinet members try to overthrow the President).