It has been noted that Heaven refers to a place in the afterlife, but in this case, Heaven means more than that. The word Heaven can also refer to the Being or beings in Heaven; similar to how, in the press, the phrases "the White House announcement" or "Scotland Yard's policies" or "Pyongyang's military brass" could refer to an official statement by an American President or cabinet, or the rules of London's police department, or the military advisors of North Korea's government.
Thus, the word "Heaven", in this context, most likely refers to G-d himself. Also, consider this quote from the ancient Chinese philosopher Mozi. Here, the Chinese word Tian is translated "Heaven", but you can tell that he was referring to the Deity.
Moreover, I know Heaven loves men dearly not without reason. Heaven ordered the sun, the moon, and the stars to enlighten and guide them. Heaven ordained the four seasons, Spring, Autumn, Winter, and Summer, to regulate them. Heaven sent down snow, frost, rain, and dew to grow the five grains and flax and silk that so the people could use and enjoy them. Heaven established the hills and rivers, ravines and valleys, and arranged many things to minister to man's good or bring him evil. He appointed the dukes and lords to reward the virtuous and punish the wicked, and to gather metal and wood, birds and beasts, and to engage in cultivating the five grains and flax and silk to provide for the people's food and clothing. This has been so from antiquity to the present.
Regarding capitalization, the word "God" is always capitalized in a monotheistic context. Thus, "God", "Allah", "Jehovah", and so on. The words "heaven" and "hell" and "purgatory" can be capitalized, as they are proper nouns, the names of specific places, but people frequently fail to capitalize them. It really doesn't matter. On the other hand, people seldom fail to capitalize the name of the Supreme Being.