A plant comprising: a gas turbine; a heat recovery unit that includes a primary heat recovery steam generator in which a primary heat exchanging unit is configured to generate primary steam by exchanging heat that is thermal energy of flue gas from the gas turbine, and a secondary heat recovery steam generator installed independently from the primary heat exchanging unit, and in which a secondary heat exchanging unit is configured to generate secondary steam by exchanging heat that is thermal energy of the flue gas having exchanged heat in the primary heat exchanging unit included in the primary heat recovery steam generator; a primary steam turbine that is configured to be driven by the primary steam generated by the primary heat recovery steam generator; a CO 2 recovery unit that includes a reboiler for recovering CO 2 contained in the flue gas discharged from the heat recovery unit; and a first reboiler heat supply line (L 21) via which the secondary steam generated by the secondary heat recovery steam generator is guided to the reboiler, characterized in that the plant further comprises: a condensed water branching line (L 19) that is branched from a condensed steam line for discharging condensed water from the reboiler; an external heat source that is installed on the condensed water branching line (L 19), and that is configured to convert the condensed water to steam; and a second reboiler heat supply line (L 23) via which steam obtained in the external heat source is guided to the first reboiler heat supply line (L 21). a gas turbine; a heat recovery unit that includes a primary heat recovery steam generator in which a primary heat exchanging unit is configured to generate primary steam by exchanging heat that is thermal energy of flue gas from the gas turbine, and a secondary heat recovery steam generator installed independently from the primary heat exchanging unit, and in which a secondary heat exchanging unit is configured to generate secondary steam by exchanging heat that is thermal energy of the flue gas having exchanged heat in the primary heat exchanging unit included in the primary heat recovery steam generator; a primary steam turbine that is configured to be driven by the primary steam generated by the primary heat recovery steam generator; a CO 2 recovery unit that includes a reboiler for recovering CO 2 contained in the flue gas discharged from the heat recovery unit; and a first reboiler heat supply line (L 21) via which the secondary steam generated by the secondary heat recovery steam generator is guided to the reboiler, characterized in that the plant further comprises: a condensed water branching line (L 19) that is branched from a condensed steam line for discharging condensed water from the reboiler; an external heat source that is installed on the condensed water branching line (L 19), and that is configured to convert the condensed water to steam; and a second reboiler heat supply line (L 23) via which steam obtained in the external heat source is guided to the first reboiler heat supply line (L 21). a condensed water branching line (L 19) that is branched from a condensed steam line for discharging condensed water from the reboiler; an external heat source that is installed on the condensed water branching line (L 19), and that is configured to convert the condensed water to steam; and a second reboiler heat supply line (L 23) via which steam obtained in the external heat source is guided to the first reboiler heat supply line (L 21).